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Stratospheric solar geoengineering without ozone loss - PubMed

  • ️Fri Jan 01 2016

Stratospheric solar geoengineering without ozone loss

David W Keith et al. Proc Natl Acad Sci U S A. 2016.

Abstract

Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of -1 W⋅m-2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y-1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.

Keywords: atmospheric chemistry; climate change; climate engineering; geoengineering; stratospheric ozone.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.

Particle aggregation, spatial distribution and chemistry. All plots represent annual average conditions resulting from a 5.6-Tg⋅y−1 steady-state injection of calcite. (Left) The fraction of solid particle mass per sectional bin vs. number of monomers in the fractal aggregate. (Middle) Particle number density (cm−3) as a function of latitude and altitude. (Right) Composition of solid particles resulting from reaction with acids showing total (black line) and CaCO3, CaCl2, Ca(NO3)2, and CaSO4 mixing ratios (parts per billion by volume) averaged from 60°S to 60°N.

Fig. 2.
Fig. 2.

Changes in ozone chemistry and distribution. All plots show changes resulting from a 5.6-Tg⋅y−1 steady-state injection of calcite. (Left) Fraction of ozone loss caused by various catalytic cycles as a function of altitude, averaged from 60°S to 60°N for annual average conditions (see Table S1 for definitions of the catalytic cycles). (Middle) Annual average change in ozone (1012 molecules cm−3) as a function of latitude and altitude. (Right) Change in column ozone (percent) as a function of latitude and season.

Fig. S1.
Fig. S1.

Particle aggregation, spatial distribution, and chemistry as a function of calcite injection rate. All plots represent annual average conditions. Rows show results from steady-state injection of calcite at rates of (from top to bottom) 0.3, 1, 2.8, and 5.6 Tg⋅y−1. (Left) Fraction of solid particle mass per sectional bin vs. number of monomers in the fractal aggregate. (Middle) Particle number density (cm−3) as a function of latitude and altitude. (Right) Composition of solid particles resulting from reaction with acids, showing total (black line) and CaCO3, CaCl2, Ca(NO3)2, and CaSO4 mixing ratios (parts per billion by volume) averaged annually and from 60°S to 60°N.

Fig. S2.
Fig. S2.

Changes in ozone chemistry and distribution. Rows show results from steady state injection of calcite at rates of (from top to bottom) 0.3, 1, 2.8, and 5.6 Tg⋅y−1. (Left) Fraction of ozone loss caused by various catalytic cycles as a function of altitude, averaged from 60°S to 60°N for annual average conditions (see Table S1 for definitions of the catalytic cycles). (Middle) Annual average change in ozone (percent) as a function of latitude and altitude. (Right) Change in column ozone (percent) as a function of latitude and season.

Fig. 3.
Fig. 3.

Trade-off between ozone loss and radiative forcing from geoengineering. Change in annual average global column ozone is plotted versus the computed aerosol radiative forcing. Changes in ozone are computed with respect to a 2040 baseline. Positive values represent an increase in ozone. Results for calcite injection rates ranging from 0.3 Tg⋅y−1 to 5.6 Tg⋅y−1 shown (red line and symbols) where the gas−solid reaction rates, γ, are varied to explore parameter uncertainty. For comparison, we show prior results for injection of alumina, diamond, and sulfate, either from injection of gas-phase SO2 or sulfuric acid (2).

Fig. S3.
Fig. S3.

Change in particle composition as a function of injection rate. (Left) CaCO3 and Ca(NO3)2, which dominate particle composition. (Right) CaCl2 and CaSO4. (Top) The global burden (in teragrams) of each species as a function of injection rate. (Bottom) The percentage of total Ca in the given species. As a rough sensitivity test, results are shown for three different values of γ, the dimensionless reaction rate.

Fig. S4.
Fig. S4.

Sensitivity of NOy, Cly, and O3 to injection rate and reaction probabilities (γ) on calcite particles. (Top) Sensitivity to injection rate (with γ = 1.0 in all cases). (Bottom) Sensitivity to reaction probability (with injection rate of 5.6 Tg⋅y−1 in all cases). Vertical profiles of (Left) NOy and (Middle) Cly mixing ratios, and (Right) O3 number density change averaged annually from 60°S to 60°N.

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